European XFEL – Page 2 – Lightsources.org

How Molecules Break and Form Bonds

2025/08/062025/08/07

Researchers at European XFEL in Germany have tracked in real time the movement of individual atoms during a chemical reaction in the gas phase. Using extremely short X-ray flashes, they were able to observe the formation of an iodine molecule (I₂) after irradiating diiodomethane (CH₂I₂) molecules by infrared light, which involves breaking two bonds and forming a new one. At the same time, they were able to distinguish this reaction from two other reaction pathways, namely the separation of a single iodine atom from the diiodomethane, or the excitation of bending vibrations in the bound molecule. The results provide new insights into fundamental reaction mechanisms that have so far been very difficult to distinguish experimentally.

So-called elimination reactions in which small molecules are formed from a larger molecule are central to many chemical processes—from atmospheric chemistry to catalyst research. However, the detailed mechanism of many reactions, in which several atoms break and re-form their bonds, often remains obscure. The reason: The processes take place in incredibly short times—in femtoseconds, or a few millionths of a billionth of a second.

An innovative experimental approach was now used at the SQS instrument at European XFEL to visualize such reaction dynamics. The researchers irradiated diiodomethane molecules with ultrashort infrared laser pulses, which triggered the molecular reactions. Femtoseconds later, intense X-ray flashes shattered the molecules, causing their atomic components to fly apart in a “Coulomb explosion.” The trajectories and velocities of the ions were then recorded by a detection device called the COLTRIMS reaction microscope (COLd Target Recoil Ion Momentum Spectroscopy)—one of the detection instruments at the SQS experimental station that is made available to users.

“Using this method, we were able to precisely track how the iodine atoms assemble while the methylene group is cleaved off,” explains Artem Rudenko from Kansas State University, USA, the principal investigator of the experiment. The analysis revealed that both synchronous and asynchronous mechanisms contribute to the formation of the iodine molecule—a result that was supported by theoretical calculations.

Remarkably, “Although this reaction pathway only accounts for about ten percent of the resulting products, we were able to clearly distinguish it from the other competing reactions,” explains Rebecca Boll from the European XFEL’s SQS (Small Quantum Systems) instrument in Schenefeld near Hamburg. This was made possible by the precise selection of specific ion fragmentation channels and their time-resolved analysis.

2014 Nobel Prize idea used to reach super-resolution

2025/07/162025/07/16

In a leap forward for atomic-scale imaging, researchers have introduced a novel X-ray technique that could transform our understanding of electron motion at the microscopic level. This cutting-edge method, developed by an international team of scientists, uses the unique properties of European XFEL at Schenefeld near Hamburg, Germany—the largest X-ray laser in the world—to capture detailed snapshots of atomic interactions. The results of this research were now published in Nature.

The technique, called stochastic Stimulated X-ray Raman Scattering (s-SXRS), turns noise into valuable data, offering snapshots of the electronic structures of atoms. This advancement sets the stage for breakthroughs in chemical analysis and materials science.

Researchers from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, the Max Planck Institute for Nuclear Physics, of European XFEL and others developed this innovative approach to X-ray spectroscopy, achieving unprecedented detail and resolution.

“For a long time, chemists have dreamed of seeing how electrons move when they’re in excited states, as these movements are what drive chemical reactions,” says Linda Young, an Argonne Distinguished Fellow and professor at the University of Chicago. “Our technique brings us closer to realizing that dream.”

The key innovation is a super-resolution technique that greatly improves the detail in X-ray spectroscopy, a method for studying electron placement around atomic centres. This advancement helps scientists identify closely spaced energy levels in atoms, offering a clearer view of their electronic structures, which determine chemical properties.

“Think of it like upgrading from a standard-definition television to an ultra-high-definition screen,” Young explains. “We’re now able to see the fine details of electronic motion that were previously blurred or invisible.”

The practical applications of stochastic Stimulated X-ray Raman Scattering are wide-ranging. For example, it can provide insights into how chemical bonds form or break, offering a deeper understanding of fundamental processes relevant to chemical analysis. This knowledge is essential for developing new materials with specific electronic properties, impacting industries like electronics and nanotechnology.

The researchers directed the X-ray pulses of European XFEL through neon gas and used a spectrometer to collect the resulting radiation. The small, 5-millimeter gas cell was designed by the Max Planck Institute for Nuclear Physics The intense beam created tiny holes in the cell’s entrance and exit windows, allowing the X-rays to pass through to a grating spectrometer—a device that separates light into its different wavelengths—provided by collaborators from Uppsala University in Sweden. The European XFEL experts have taken on a vital role in coordinating the installation and performing thorough pre-experimental testing. “This ensured optimal focusing conditions, which were crucial for efficiently acquiring a large amount of data during the experiment” explains Michael Meyer, group head of the Small Quantum Systems (SQS) instrument at European XFEL and a researcher in the Cluster of Excellence ‘CUI: Advanced Imaging of Matter’.

As the X-rays pass through the gas, they amplify the Raman signals—a type of X-ray fingerprint that provides information about the excited electronic states of atoms or molecules—by nearly a billion-fold. This amplified signal provides detailed information about the electronic structure of the gas on a femtosecond timescale, or one quadrillionth of a second. By analysing the relationship between the incoming pulses and the resulting Raman signals, scientists can create a detailed energy spectrum from many individual snapshots, rather than scanning slowly across different energy levels.

“The large number of pulses in each X-ray flash not only boosts the measurement signal but also holds the key to the highest spectral resolution by averaging over many photon impacts on the detector at once,” says Thomas Pfeifer from the Max Planck Institute for Nuclear Physics.

“This approach, pinpointing the centre position of broad but distinct spectral spikes much more precisely than the width of the spikes, is similar to the super-resolution microscopy technique that won the 2014 Nobel Prize in chemistry”, Pfeifer adds.

Image: An incoming X-ray light wave (left) made up of a chaotic distribution of very fast spikes interacts with atoms (purple dots) in a gas to amplify specific spikes (right) in the light wave.

Credit: Illustration by Stacy Huang/Argonne National Laboratory

Structure of liquid carbon measured for the first time

2025/05/202025/05/22

With the declared aim of measuring matter under extreme pressure, an international research collaboration headed by the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) used the high-performance laser DIPOLE 100-X at European XFEL for the first time in 2023. With spectacular results: In this initial experiment they managed to study liquid carbon – an unprecedented achievement, as the researchers report in the journal Nature (DOI: 10.1038/s41586-025-09035-6).

Liquid carbon can be found, for example, in the interior of planets and plays an important role in future technologies like nuclear fusion. To date, however, only very little was known about carbon in its liquid form because in this state it was practically impossible to study in the lab: Under normal pressure carbon does not melt but immediately changes into a gaseous state. Only under extreme pressure and at temperatures of approximately 4,500 degrees Celsius – the highest melting point of any material – does carbon become liquid. No container would withstand that.

Laser compression, on the other hand, can turn solid carbon into liquid for fractions of a second. And the challenge was to use these fractions of a second to take measurements. In a previously unimaginable way, this has now become reality at the European XFEL, the world’s largest X-ray laser with its ultrashort pulses, in Schenefeld, near Hamburg.

Unique measuring technology in this combination

The unique combination of the European XFEL with the high-performance laser DIPOLE100-X was crucial for the success of the experiment. It was developed by the British Science and Technology Facilities Council and made available to scientists from all over the world by the HIBEF User Consortium (Helmholtz International Beamline for Extreme Fields). A community of leading international research institutions at the HED-HIBEF (High Energy Density) experimental station at European XFEL has now combined powerful laser compression with ultrafast X-ray analysis and large-area X-ray detectors for the first time.

In the experiment, the high-energy pulses of the DIPOLE100-X laser drive compression waves through a solid carbon sample and liquefy the material for nanoseconds, that is, for a billionth of a second. During this nanosecond, the sample is irradiated with the ultrashort X-ray laser flash of the European XFEL. The carbon atoms scatter the X-ray light – similar to the way light is diffracted by a grating. The diffraction pattern allows inferences to be drawn about the current arrangement of the atoms in the liquid carbon.

The whole experiment only lasts a few seconds but is repeated many times: every time with a slightly delayed X-ray pulse or under slightly different pressure and temperature conditions. Many snapshots combine to make a movie. Researchers have thus been able to trace the transition from solid to liquid phase one step at a time.

Image: Groundbreaking experiment at European XFEL: Research team measured structure of liquid carbon for the first time

Credit: Martin Kuensting / HZDR

A pioneering spectrometer for hard X-rays

2025/04/162025/04/24

Researchers at the European XFEL have developed a new device for X-ray measurements at high photon energies–a so-called Laue spectrometer. It enables X-ray light with photon energies of over 15 kiloelectronvolts to be detected with improved efficiency and highest precision. This is important for researching technically significant materials that, for example, transport electricity without losses or ensure that chemical processes run more efficiently.

To unravel the secrets of the world of atoms, molecules and materials in general, scientists often use special measurement devices known as spectrometers. They work by recording the light that objects emit. From the way in which the objects do that, researchers learn a lot about the physical processes that take place in the materials. Particularly revealing is the research with X-ray light, which penetrates deeply in matter and provides information specific to each atomic species. This light is invisible to our eyes, but can be detected and measured using special spectrometers.

The main components of these devices are usually extremely precisely cut crystals made of silicon or germanium. Traditionally, the X-ray spectrometers work in what is known as Bragg geometry: The X-ray light hits the crystal and is then diffracted by the atomic planes parallel to the surface, similarly as mirrors reflect visible light. From the direction and intensity of the scattered radiation, the researchers can draw conclusions about the electronic properties of the materials they are analysing.

A unique characteristic of European XFEL is the ability to provide X-ray light with very high energy. However, as the energy of the X-rays increases, the interaction with the crystals becomes smaller, making the measurements challenging. In this high photon energy regime, a large proportion of the X-ray light simply passes through the crystal unused, which is why the performance of X-ray spectrometers using these analysers, known as Johann or Von Hamos spectrometers, decreases rapidly with increasing X-ray energy. They usually only work well up to a photon energy of around 15 kiloelectronvolts (keV).

Image: Contrary to regular spectrometer the new Laue spectrometer is diffracting the X-ray beams (red arrows) by atomic layers perpendicular to the surface.

X-ray snapshot: How light bends an active substance

2025/03/102025/03/12

With the help of the world’s most powerful X-ray laser, European XFEL, a research team led by Goethe University Frankfurt and the research centre DESY has achieved an important breakthrough: Using the example of the pharmaceutically active substance 2-thiouracil, they applied a long-established imaging technique to complex molecules for the first time. Although 2-thiouracil is no longer applied therapeutically, it is part of a group of chemically similar active substances that are used today as immunosuppressants or cytostatics. The study shows how UV radiation deforms 2-thiouracil, making it dangerously reactive.

Many biologically important molecules change shape when stimulated by UV radiation. Although this property can also be found in some drugs, it is not yet well understood. Using an innovative technique, an international team involving researchers from Goethe University Frankfurt, the European XFEL in Schenefeld and the Deutschen Elektronen-Synchrotron DESY in Hamburg has elucidated this ultra-fast process, and made it visible in slow motion, with the help of X-ray light. The method opens up exciting new ways of analysing many other molecules.

“We investigated the molecule 2-thiouracil, which belongs to a group of pharmaceutically active substances based on certain DNA building blocks, the nucleobases,” says the study’s last author Markus Gühr, the head of DESY’s free-electron laser FLASH and Professor of Chemistry at University of Hamburg. 2-thiouracil and its chemically related active substances have a sulphur atom, which gives the molecules its unusual, medically relevant properties. “Another special feature is that these molecules become dangerously reactive when exposed to UV radiation.” Studies indicate an increased risk of skin cancer due to this effect.

To better understand what happens during such processes, the research team used an already well-established method, bringing it to a new level by applying the technical possibilities available today. “Coulomb explosion imaging involves irradiating a molecule with intense X-ray pulses, which knock out electrons,” explains Till Jahnke, Professor of Experimental Atomic and Molecular Physics at Goethe University and the study’s first author. “Thereby, the molecule charges up positively and thus becomes unstable, so that it is torn apart within fractions of a second.” By tracking the direction in which the various fragments of the molecule – the atoms – fly apart, it is possible to derive information about the molecule’s structure.

Image: The SQS instrumentâ€™s COLTRIMS reaction microscope was used to analyze the structural changes of the 2-thiouracil molecule at the European XFEL.

Credit: European XFEL

On the hunt for axions

2025/02/192025/02/21

New X-ray experiment at the European XFEL could solve some of the mysteries of physics

Researchers at European XFEL, together with colleagues from the UK Science and Technology Facilities Council (STFC), the University of Oxford and other research institutions, have been searching for a hypothetical particle that could potentially explain the dark matter of the universe. The experiment is described in a study published in Physical Review Letters.

The researchers hunted for so-called axions at the High Energy Density instrument HED/HiBEF at European XFEL. Axions are tiny and incredibly light hypothetical particles. They are intended to explain, for example, why neutrons, which make up atomic nuclei alongside protons, have no electric dipole moment, even though the nuclear building blocks consist of even smaller charged particles known as quarks. This could also be an indication of new physics beyond the standard model. Furthermore, axions are a natural candidate for dark matter, the mysterious substance that makes up most of the structure of the universe.

The researchers used European XFEL in Schenefeld near Hamburg, the largest and most powerful X-ray laser in the world for their experiments. They channelled the intense X-ray beam of European XFEL through thin plates of germanium crystals. These have strong electric fields inside. For moving particles, this appears like an extremely strong magnetic field of around 1000 Tesla. This enables photons to transform themselves into axions and back again.

Image: Axion search at the HED/HiBEF instrument of European XFEL

Credit: European XFEL

Young Scientist Award for Patrick Heighway

2025/01/222025/01/23

“Patrick Heighway deserves the prestigious prize for his pivotal role in measuring X-ray diffraction at extreme pressures and temperatures at the HED-HiBEF Instrument”, says Emma McBride from Queen’s University, Belfast and chairperson of the User Organization Executive Committee (UOEC).

His work combines experimental data with molecular dynamics simulations to provide critical insight into the nature of release pathways of shock compressed materials, kinematics of plasticity, and the fundamental interaction of grains in compressed polycrystalline materials. This work is important for many different fields, including geophysics, fundamental material science, shock and plasma physics, the search for novel materials, and understanding pathways to fusion energy.

The European XFEL Young Scientist Award recognises young researchers who are at the beginning of their career but are already making outstanding contributions to research at the European XFEL.

The winner will receive a monetary award of 2,000 Euro and was invited to give a talk as part of the plenary session of the European XFEL User Meeting on 21 January 2025 in Hamburg.

For the first time, the European XFEL User Organization Executive Committee awards as well prizes to posters presented this year at the European XFEL Users’ Meeting about exciting research performed with radiation from European XFEL. Poster prizes were awarded to Daniele Ronchetti (CFEL), Calum Prestwood and Carolina Camarda (both European XFEL).

Their topics were “Elastic scattering enhancement via transient resonances” (Ronchetti), “Tracking atomic populations and transitions in x-ray heated mid-Z transition metals” (Prestwood), and “Electronic properties of Ferropericlase (Mg,Fe)O obtained from dynamic compression experiments using DiPOLE100-X at European XFEL” (Camarda).

Ryszard Sobierajski new Council vice-chair

2024/12/122024/12/18

At the recent meeting of the European XFEL Council, Dr hab. Ryszard Henryk Sobierajski was elected as new vice-chair of the European XFEL’s highest governing body. He will follow by the end of the year Prof. Dr James (Jim) Henderson Naismith.

“We thank Jim for many years of inspiring contributions as European XFEL’s vice-chair,” says Thomas Feurer, Managing Director and Chair of the Management Board of European XFEL. “And we heartily welcome our well-known colleague Ryszard.”

“Ryszard is a profound expert of research with synchrotron light and free electron-lasers, and an experienced science manager,” adds Federico Boscherini, Chair of the European XFEL Council.

Sobierajski takes up his office with effect from 1 January 2025 and for a period of two years. He is Associate Professor at the Institute of Physics of the Polish Academy of Sciences, Warszawa, Poland and since January 2020 one of the Polish delegates to the European XFEL council. Additionally, he is an expert on the Proposal Review Panel for the HED instrument at European XFEL–most of the time as its chair.

15 years of European XFEL

2024/11/292024/11/29

European XFEL, one of the world’s most powerful X-ray sources, is celebrating the 15th anniversary of the international treaty that laid the foundation for its creation this year. On 30 November 2009, ten European countries jointly decided to implement the ambitious research project and create an internationally accessible research facility that would offer new, unparalleled research opportunities to scientists from all over the world.

“European XFEL has become a symbol of successful scientific collaboration across national borders,” says Thomas Feurer, Managing Director and Chairman of the Management Board of European XFEL.

The X-ray laser, whose first light beam was generated in 2017, has since enabled ground-breaking research worldwide. Researchers from disciplines like physics, chemistry, biology, medicine and materials science benefit now from the facility at seven instruments, whose intense X-ray light beam offers unique insights into the molecular structure of matter and dynamic electronic or chemical processes in real time. Thanks to its high beam power, molecular structures and chemical reactions can be observed with unrivalled precision and speed, far exceeding conventional technologies. Most recently, researchers were able to show that the European XFEL can generate record-breaking X-ray pulses in the attosecond range with terawatt power.

The construction of the facility was supported by strong partnerships right from the start: the close collaboration with the Deutsches Elektronen-Synchrotron (DESY) in Hamburg played a decisive role in the realisation and operation of the European XFEL.

Image: Ministers, state secretaries and other government representatives from ten partner countries met in November 2009 in the Hamburg City Hall to sign the international European XFEL agreement.

Credit: European XFEL

Squeeze it! High-power attosecond X-ray pulses at megahertz repetition rates

2024/11/242024/11/25

A research team at European XFEL and DESY has achieved a major advance in X-ray science by generating unprecedented high-power attosecond hard X-ray pulses at megahertz repetition rates. This advancement opens new frontiers in the study of ultrafast electron dynamics and enables non-destructive measurements at the atomic level.

Researchers have demonstrated single-spike hard X-ray pulses with pulse energies exceeding 100 microjoules and pulse durations of only a few hundred attoseconds. An attosecond is one quintillionth (10^-18) of a second—a timescale that allows scientists to capture even the fastest electron movements in matter.

“These high-power attosecond X-ray pulses could open new avenues for studying matter at the atomic scale,” says Jiawei Yan, physicist at European XFEL and lead author of the study published in Nature Photonics. “With these unique X-rays, we can perform truly damage-free measurements of structural and electronic properties. This paves the way for advanced studies like attosecond crystallography, allowing us to observe electronic dynamics in real space.”

Traditional methods for generating such ultra-short hard X-ray pulses required dramatically reducing the electron bunch charge to tens of picocoulombs, which limited the pulse energy and practical use. The team developed a self-chirping method, utilizing the collective effects of electron beams and specialized beam transport systems at the European XFEL. This approach enables the generation of attosecond X-ray pulses at terawatt-scale peak power and megahertz repetition rates without reducing the electron bunch charge.

“By combining ultra-short pulses with megahertz repetition rates, we can now collect data much faster and observe processes that were previously hidden from view”, says Gianluca Geloni, group leader of the FEL physics group at the European XFEL. “This development promises to transform research across multiple scientific fields, especially for atomic-scale imaging of protein molecules and materials and investigating nonlinear X-ray phenomena.”

Image: Scientists at European XFEL and DESY produce high-power attosecond X-ray pulses at megahertz repetition rates. With the help of special beam optics relativistic electrons (blue cloud) are strongly compressed (bright line in the centre). This leads to a very bright, high-power X-ray pulse on the attosecond timescale.

Credit: European XFEL; Illustration: Tobias Wüstefeld

European XFEL opens modern exhibition and conference centre

2024/11/202024/11/21

Schenefeld, 20.11.2024 – Together with high-ranking guests, European XFEL today opens the modern Lighthouse exhibition and conference centre on its campus in Schenefeld near Hamburg to the public. The two-storey building offers space for a 350 m² permanent exhibition, 200 m² of special exhibition space, the Xcool Lab with two laboratories for students, and rooms for conferences and events. The name Lighthouse was suggested by the staff.

The new Lighthouse exhibition and conference centre of European XFEL offers a fascinating scientific experience. Together with the DESY visitor centre DESYUM, which is due to open in 2025, it will take visitors on an even more comprehensive journey of discovery into modern research with X-ray light sources and particle physics.

Guido Wendt, State Secretary for Science, Research and Culture, Schleswig-Holstein: “European XFEL enables cutting-edge international research, with outstanding experiments and brilliant results that inspire the global scientific community. We also want to communicate this to the public – and especially to schoolchildren. The new exhibition and conference centre with its two laboratories for schoolchildren will provide us with excellent support in the future.”

Eva Gümbel, State Councilor, Authority for Science, Research, Gender Equality and Districts of Hamburg: “At the European XFEL, researchers from all over the world carry out unique experiments and develop new research opportunities. With the new exhibition and conference centre, this can be experienced directly by students: through interactive exhibits, original pieces and multimedia presentations. The combination of excellent research and knowledge transfer is a real benefit for our science location and a great experience for all visitors.”

Schenefeld’s mayor Christiane Küchenhof: “Every lighthouse is unique, but the town of Schenefeld now has the most unique lighthouse in the world. The visitor and conference centre with this beautiful name will now share its light with many guests. I am delighted about this important new attraction on the Schenefeld science campus.”

Image: The cutting of the red ribbon marks the official opening of the Lighthouse exhibition and conference centre (from left to right: Nicole Elleuche, Helmut Dosch, Eva Gümbel, Volkmar Dietz, Christiane Küchenhof, Guido Wendt, Federico Boscherini, Thomas Feurer)

Credit: European XFEL / Axel Heimken

Thomas Feurer elected as future Chairman of LEAPS

2024/10/252024/10/29

At their annual meeting, the 16-member organisations of the League of European Accelerator-based Photon Sources (LEAPS) elected Prof. Dr Thomas Feurer, Chair of the Management Board of European XFEL, as their future chairman. LEAPS is a strategic initiative that brings together major European synchrotron radiation and free electron laser (FEL) facilities. Through joint efforts, LEAPS seeks to advance photon science and to maximize the impact of accelerator-based light sources in Europe. Feurer will succeed Jakub Szlachetko from the National Synchrotron Radiation Centre SOLARIS, Krakow (Poland). The handover will take place at the next plenary session in October or November next year. Until then, he will enjoy the status of Incoming Chair.

Image: Thomas Feurer from European XFEL elected Chairman of LEAPS

Credit: European XFEL

Congratulations to the Nobel Prize winners in chemistry

2024/10/112024/10/11

The researchers at the world’s largest free-electron laser, the European XFEL, are delighted that Demis Hassabis, John M. Jumper and David Baker have been awarded the Nobel Prize in Chemistry. The decoding of protein structures is an important field of research for X-ray lasers such as the European XFEL.

David Baker has been an active user of the European XFEL since 2022. His team has actively participated in single-molecule imaging experiments at the Small Quantum Systems (SQS) and SPB/SFX instrument.

There, they recorded diffraction patterns of computationally designed proteins and single molecules for the first time.

“We are excited that David Baker has received the Nobel Prize for his ground-breaking work in the computer-aided design of de novo proteins”, says Thomas Feurer, Chairman of the Management Board of European XFEL. “We look forward to collaborating on upcoming experiments where he plans to explore the ultra-fast dynamics and behaviour of these innovative proteins with us.”

Image: David Baker, Demis Hassabis and John Jumper. Ill. Niklas Elmehed

Credit: Nobel Prize Outreach

European XFEL creates exotic matter

2024/07/292024/07/29

Exploring the extreme conditions reached in the interior of planets, including Earth, or during a fusion reaction, is a major challenge. By focusing the extremely powerful X-ray laser of European XFEL on a copper foil, researchers have created and investigated a state of matter very far from equilibrium, coined warm dense matter (WDM), that resembles such exotic environments. Their findings make remarkable strides in understanding and characterizing this elusive state of matter, which is crucial for advancing inertial confinement fusion, a process that holds promise for clean and abundant energy.

Heat can drastically change the state of matter: depending on the temperature, substances are solid, liquid or gaseous. In a certain temperature range, matter also assumes a state known as warm dense matter (WDM): it is too hot to be described by the physics of condensed matter, but at the same time too dense for the physics of weakly coupled plasmas. The boundary between warm dense matter and other states of matter is not precisely defined. Often a temperature range of 5,000 Kelvin to 100,000 Kelvin is specified at pressures of several hundred thousand bar, whereby one bar corresponds to the air pressure on Earth surface. WDM is not stable in our daily environment and is very difficult to produce or even examine in the laboratory. Typically, scientists compress samples in diamond anvil cells to reach high pressures, or use powerful optical lasers to turn solids into WDM for a tiny fraction of a second.

The intense X-ray pulses of European XFEL have now proved to be a very useful tool for generating and analysing warm dense matter. The researchers used copper as a sample material. “The high intensity of the pulses can excite the electrons in the copper foil to such an extent that it switches to the state of warm dense matter,” explains Laurent Mercadier, a scientist at the SCS[1] instrument who led the experiment: “This can be seen in a change in its light transmission.”

A metal that is irradiated by an intense X-ray pulse can become transparent if the electrons in the metal absorb X-ray energy so fast that there are no electrons left to excite. The remaining tail of the pulse can then penetrate the material unhindered. This is known as saturable absorption (SA). Conversely, a metal can become increasingly opaque if the front of the pulse creates excited states that have higher absorption coefficient than the cold metal. The tail of the pulse is then absorbed stronger, an effect known as reverse saturable absorption (RSA). Both processes are routinely used in optics, for example to generate a specific pulse length with lasers.

Image: Laurent Mercadier checks the setup in the experimental chamber

Credit: European XFEL

Thomson scattering: Reaching unmatched level

2024/07/112024/07/11

Researchers at European XFEL have developed an innovative method to study warm dense matter with unprecedented accuracy. This kind of matter, that exists between condensed matter and plasma physics, can be found, for example, in astrophysical objects or is created during inertial confinement fusion. For the contributing scientists at the Center for Advanced Systems Understanding (CASUS), this advancement is a great aid to their mission of lifting the analysis of warm dense matter onto a solid foundation.

Studying astrophysical objects is a major challenge. Extreme conditions prevail there: high temperatures and immense densities. Here on Earth, the same applies to the investigation of inertial confinement fusion capsules during the implosion phase. At the High Energy Density (HED) instrument of European XFEL these conditions can be prepared there using the powerful drivers provided by the HiBEF consortium (Helmholtz International Beamline for Extreme Fields). Coupled with the brilliant X-ray flashes of the European XFEL, scientists can now study this exotic state of matter more closely than ever before.

Warm dense matter: an exceptional phenomenon

We normally think of matter here on Earth as existing in either a solid, liquid or gaseous state. Further afield in space, matter existing as a plasma can also be discovered, characterized as a hot and ionized gas. However, at high temperatures and immense densities, like that found in stars or when meteors crash onto planets, matter cannot be easily described as solid, or as a plasma and is instead named warm dense matter. Warm dense matter is too hot to be described by the physics of condensed matter and too dense for the physics of plasma. Typically, warm dense matter occurs at temperatures of 5,000 to several 100,000 Kelvin and pressures of several hundred thousand times greater than atmospheric pressure.

Discovery thanks to ultra-high-resolution X-ray Thomson scattering

A team led by Thomas Preston from the HED instrument at European XFEL has investigated the structure and properties of plasmons in ambient aluminum. Plasmons are collective oscillations of electrons and play a decisive role in the optical properties of metals, semiconductors, and in warm dense matter. An important method to investigate excitations in solids as well as warm dense matter is X-ray Thomson scattering. Here an X-ray photon is scattered in the material and loses energy and momentum in exciting a plasmon. With a spectrometer, scientists can identify these photons that have lost energy from the main beam of X-rays that are just scattered elastically.

Differently to previous work, which could only measure these excitations with X-rays with poor resolution on the order of a few electronvolts, Preston’s team and contributing scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) as well as the HZDR institute CASUS have now recorded ultra-high-resolution X-ray Thomson scattering spectra with an energy resolution improved more than tenfold meaning that they reached a resolution of less than one hundred millielectronvolt.

The team have published their findings recently in the journal Physical Review B, which honored the work with an “Editor’s Suggestion”. As the team reports, the new set-up enabled the investigation of the structure and properties of plasmons in aluminum in detail. “We realized that we could repurpose an existing setup that was designed to make even higher resolution measurements of vibrations in solids, which have energy losses much smaller than scattering from a plasmon, in fact only a few tens of millielectronvolts,” explains Preston. “Through a clever choice of our X-ray energy, we can instead measure energy losses up to 40 electronvolts with similar resolution. The accuracy of our measurements made it possible to eliminate long-standing discrepancies between simulations and experimental observations,” describes Preston. In future work, the team intend to use this method to benchmark simulations for plasmons at higher temperatures and compressions.

Image: The head of the HED instrument, Ulf Zastrau, assembling components in Interaction Chamber 1, where the experiments were carried out.

Credit: European XFEL

Frozen noble gas in the accelerator

2024/05/242024/06/05

Researchers at European XFEL in Schenefeld near Hamburg have taken a closer look at the formation of the first crystallisation of nuclei in supercooled liquids. They found: The formation starts much later than previously assumed. The findings could help to better understand the creation of ice in clouds in the future and to describe some processes inside the Earth more precisely.

Every child knows that water freezes into ice when it gets icy cold. For water, this normally happens below zero degrees Celsius, the melting temperature of water. This is a fixed point on the Celsius temperature scale that we use.

However, the transition from the liquid to the solid phase is a very complex process and is difficult to study experimentally at the atomic level. One reason for this is that crystals are formed randomly: You don’t know exactly when and where it will happen. Furthermore, a liquid can remain in a metastable state for a long time: It remains liquid even though it should actually freeze and become solid. This makes it extraordinarily difficult to pinpoint the right moment for a crystal to form and watch its growth.

However, these effects are highly relevant in nature. For example, they play a decisive role in the formation of ice in clouds or in processes inside Earth.

Using the intense X-ray flashes of the European XFEL’s X-ray free-electron laser, an international team of researchers at the European XFEL in Schenefeld near Hamburg has now succeeded in precisely measuring the nucleation of supercooled liquids. The experiments took place in a vacuum so that the X-ray light does not interact with the molecules in the air, which would interfere with the experiments. Because of its complexity, however, water is one of the most difficult liquids to model. For that reason, the researchers used instead argon and krypton in liquid form in their experiments. In fact, supercooled noble-gas liquids are the only systems for which reliable theoretical predictions can be presently made.

The researchers explicitly investigated the so-called crystal nucleation rate J(T). This is a measure of the probability that a crystal will form in a certain volume within a certain time. The rate at which this happens is an important parameter, for example in order to be able to mathematically describe real processes in models – in weather forecasting, for example, or in climate models.

Image: X-ray of a crystal. The diffraction pattern results from 34,000 single-pulse x-ray exposures of a krypton jet shortly after the onset of crystal nucleation. The rings indicate x-ray scattering from specific molecular planes within the small crystals.

Credit: European XFEL